Multiple input, multiple output (MIMO) communication systems are known in the art. The term MIMO refers to communication systems that employ an array of antennas at both the transmitter and the receiver. A system having a single transmit antenna and two receive antennas is commonly referred to as a receive diversity system. A system having multiple transmit antennas and a single receive antenna is commonly referred to as a transmit diversity system. Transmit diversity systems commonly use space-time codes such as an Alamouti codes. A system having multiple transmit and multiple receive antennas is referred to as a MIMO system.
Space-time block coding is a well known technique used in wireless communication systems to transmit multiple representations of a data stream across a number of antennas and to exploit the various received versions of the data to improve the reliability of data-transfer. Since the transmitted data traverses a potentially difficult environment with scattering, reflection, refraction, etc. in addition to corruption by thermal noise in the receiver, some representations of the received data will be in better shape than others. This redundancy results in a higher chance of being able to use one or more of the received representations of the data to correctly decode the received signal. Space-time coding combines all copies of the received signal in an optimal way so as to extract as much information from each copy as is possible.
There are two basic motivations for using multiple antennas in a wireless communications system. The first motivation is to gain an improvement in diversity, while the second motivation is to gain an improvement in achievable data rate/capacity. Multiple transmit antennas may be used to convey either dependent data streams (to increase immunity to fading or increase coverage) or independent data streams (to increase the capacity or data rate of the system). These two motivations are illustrated using examples of two simple MIMO systems.
The first example refers to a system having a single transmit antenna and two receive antennas, such as system 10 shown in FIG. 1A. It is assumed that the channels or links between the transmit antenna and the receive antennas are independent fading channels, i.e., both links have a certain outage probability, which is defined as the probability of being in a relatively deep fade and may be practically disconnected. If the fade (i.e. outage) events are independent, then the probability that both channels fade together is smaller than the outage probability of an individual channel. Hence, the system with two receive antennas has a smaller outage probability and is therefore more reliable. The reduction in the outage probability is referred to as the diversity gain, and a system having two receive antennas has a larger diversity than the system having a single receive antenna. That is, it utilizes multiple, diverse replication of the transmitted signal.
The second motivation for the use of a multiple antenna system is the improvement in available data rate. Consider a system 12 shown in FIG. 1B where the data is conveyed over two independent channels with no inter-antenna interference. It is assumed that both communication links are identical, i.e. have the same statistical characteristics. It is noted that if each single channel can reliably convey a certain data rate then the aggregate data rate which can reliably be reconstructed at the receiver end using both channels is twice the data rate of one of the channels.
The problem, however, in the construction of feasible MIMO receivers is the fact that there is interference (i.e. cross coupling) between the two transmit/receive chains, as shown in the system 14 of FIG. 1C. Due to this fact, the MIMO receiver does not simply reduce to two independent single input, single output (SISO) receivers, thus entailing twice the computational complexity of a SISO receiver. The capacity of a practical MIMO system, however, grows linearly with the dimension of the MIMO system. The computational complexity of an optimum receiver in this case is not twice that of a SISO receiver, but is the complexity of a SISO receiver squared. As the number of dimensions increases, the detection problem becomes very complex. Assuming M-ary signaling in each dimension (transmit/receive antenna), an N-dimensional signaling vector results in MN possible transmit signals in each channel use (i.e. transmit time in which all of the dimensions are used). The exponential growth of the signaling set with the dimension of the channel model necessitates suboptimum, reduced complexity detectors.
The two advantages of MIMO systems mentioned supra, make MIMO an appealing feature in wideband wireless communication systems. Currently, there is considerable interest in MIMO systems, and a majority of the current state of the art communication standards such as 3GPP LTE, IEEE 802.11n and IEEE Std 802.16e (i.e. WiMAX) incorporate several MIMO features.
Thus, there is a need for a MIMO detection solution that is capable of providing both hard and soft information (i.e. soft value) decisions for use by a channel decoder. It is further desirable that the detection solution provide a capability of improving the quality of the soft information decisions. The MIMO detector should minimize the required computational complexity requirements while maximizing the receiver bit error rate performance.